1. Field of the Invention
The present invention relates to an apparatus and method for producing a perception of grayscale shading on a monochrome display. More specifically, the invention relates to a perceived grayscale shading apparatus and method which operates in the time domain to substantially avoid visual disturbances such as flicker, "swimming" and "movie-marquee effect".
2. Description of the Relevant Art
Many information systems and computer systems have multiple-bit grayscale graphic display capabilities in which each picture element (pixel) memory cell defined within the rows and columns of a two-dimensional display has a number of brightness levels. Many of these information systems and computer systems also utilize a monochrome graphic display that displays a single intensity graphic on a contrasting (black) background. For many applications and graphics, it is desirable to display a multiple gray shade graphic on the monochrome display in a manner which creates a perception of a grayscale graphic display.
Various techniques have been used to create this perception. In analog displays, gray levels are displayed by applying different voltage levels to the display. In other displays, pulse width modulation is used so that gray levels are furnished by varying the time for which a constant voltage is applied to a pixel. In still other displays, frame rate control techniques are utilized in which a graphic is displayed over several time frames during which a constant voltage may either be supplied or withheld. Gray scales are displayed by selectively supplying or withholding the constant voltage for each of the frames.
For example, Bassetti, Jr. et al. in U.S. Pat. No. 5,185,602 entitled "METHOD AND APPARATUS FOR PRODUCING PERCEPTION OF HIGH QUALITY GRAYSCALE SHADING ON DIGITALLY COMMANDED DISPLAYS", issued on Feb. 9, 1993, describes such a display technique. Here, the perception of grayscale shading on a digitally commanded display is produced by commanding pixels of the display with brightness-setting signals of differing average duty cycles. Brightness-setting signals having one brightness level associated with them are phase-shifted in relation to time and distributed to spaced-apart pixel locations at which one brightness level is to be produced. The energy of spatially-adjacent pixels is scattered in time and pixels which are energized at the same time are selected to be spatially scattered so as to avoid the perception of visual disturbances such as flickering and surface streaming.
Bassetti et al. utilize a signal synthesis technique in which grayscale waveform data is accessed and modulated with a phase signal synthesized from the grayscale data with the different phases spatially distributed in a phase pattern matrix. Accordingly, the grayscale data is manifest as a pattern of frames having an average duty cycle indicative of a gray shade.
In another example, Garrett J. H. in U.S. Pat. No. 5,068,649 entitled "METHOD AND APPARATUS FOR DISPLAYING DIFFERENT SHADES OF GRAY ON A LIQUID CRYSTAL DISPLAY", issued Nov. 26, 1991, teaches an alternative display technique. The system disclosed by Garrett furnishes a means for both spatially and temporally resolving the on/off states of a two-state display device to provide apparent shades of gray. Cycling between on and off states is not performed in a discernible pattern, but rather a pseudo-random pattern is utilized which repeats only after many cycles. Adjacent pixels, when selected to display the same shade of gray, do not cycle on and off in synchronization, but rather use out-of-phase cycling patterns. This spatial resolution reduces perceived flicker in the display and creates a more stable graphic.
The Garrett system uses predetermined patterns which repeat only after predetermined numbers of cycles to provide the gray scale. The pseudo-random pattern cycling is accomplished by "causing a predetermined skewing of each subsequently generated display signal having a pattern cycle for which the total number of display elements in a row is integrally divisible each time a bit of a respective display signal is provided for the last display element of a row" in accordance with Garrett's claim 1.